Magnetic resonance imaging is an image reconstruction process that involves transmitting an RF signal at a Larmor frequency into an object in a static magnetic field to magnetically excite a nuclear spin in the object and reconstructing an image from an MR signal produced as a result of the nuclear spin.
Magnetic resonance angiography (MRA) to obtain a blood flow image is a known magnetic resonance imaging method. A type of MRA that does not use a contrast medium is referred to as non-contrast MRA. For the non-contrast MRA, a fresh blood imaging (FBI) method and a steady state free precession (SSFP) method have been devised. The FBI method uses electrocardiogram gating to visualize a blood vessel with high quality by capturing a blood flow at a high flow speed pumped by a heart. The FBI method and the SSFP method produce a bright blood image in which the blood has a high signal value and is shown in white.
In the FBI method, a flow dephasing pulse or a flow rephasing pulse may be added to a readout gradient pulse in order to visualize a blood flow at a low flow speed. In this case, the flow dephasing pulse or the flow rephasing pulse serves to increase the signal difference between a signal value from a blood flow at a high flow speed and a signal value from a blood flow at a low flow speed. Since the signal difference increases, an artery and a vein can be more clearly distinguished from each other.
Furthermore, as a technique of determining an appropriate intensity of the flow dephasing pulse or the flow rephasing pulse, there has been devised a technique of performing, in advance of the imaging scan, a pre-scan referred to as flow-prep scan to gather data while changing the intensity of the flow dephasing pulse or the flow rephasing pulse.
Furthermore, in order to reduce an N/2 artifact produced as a result of a periodic variation of the signal from a blood flow in a k-space, there has been devised a gradient moment nulling (GMN) technique to eliminate a gradient moment in a readout (RO) direction. According to the GMN technique, a rephasing pulse having an appropriate intensity can be applied, so that the N/2 artifact can be effectively reduced.
The techniques described above are intended for the bright blood imaging in which the blood (or the blood flow) has a high signal level. On the other hand, a black blood imaging method that produces a blood vessel wall image in which the blood has a low signal value and is shown in black and thus the blood vessel wall is emphasized is also often used for diagnosis.
According to prior art, the bright blood imaging to acquire a blood image and the black blood imaging to acquire a blood vessel wall image are separately performed, so that imaging has to be performed at least twice to obtain both the blood image and the blood vessel wall image.
Thus, there is a need for a magnetic resonance imaging apparatus that can easily obtain a blood image and a blood vessel wall image in a short time in a non-contrast manner.